The present disclosure relates to a low frequency ultrasonic communication system for performing communication using an ultrasonic wave, a low frequency ultrasonic communication method, a low frequency ultrasonic transmitting apparatus, and a low frequency ultrasonic receiving apparatus.
Recently, an NFC and Bluetooth system has been widely used for short range communication. However, only devices each having a separate hardware system built therein are available in the NFC and Bluetooth system, and therefore, separate cost is required.
In order to solve such a problem, there is introduced a low frequency ultrasonic communication system for performing communication using a low frequency ultrasonic wave. The low frequency ultrasonic communication system has an advantage that any device having a central processing unit, a microphone, and a speaker can perform communication. Thanks to the advantage, low frequency ultrasonic communication can be performed in any space in which sound can be delivered.
Non-Patent Document 1, identified below, has been introduced as an example of the low frequency communication system. In the Non-Patent Document 1, there is introduced a low frequency ultrasonic (LFU) communication system based on amplitude shift keying (ASK), which is called “Sonicom.” However, in the Non-Patent Document 1, synchronization and modulation relies on the energy level of a carrier frequency. This is because communication relies on a channel having high frequency selectivity. For this reason, the communication is considerably weak against channel noise and attenuation. Therefore, according to the Non-Patent Document 1, there is a problem that the maximum transmission range is limited to about 2 m.
In order to solve the problem of the Non-Patent Document, Non-Patent Document 2, identified below, has been introduced. In the Non-Patent Document 2, the maximum transmission distance can be increased up 25 m, using a chirp signal as a modulation symbol. However, according to the Non-Patent Document 2, there is a problem that the transmission speed is remarkably decreased to 16 bps due to inferior modulation efficiency of the chirp signal, in terms of both time and frequency.
Under the above-described circumferences, there is required a low frequency ultrasonic communication system that is robust with respect to a noise environment and a channel variation and can secure a transmission distance of a certain level or higher.